Airbnb Swift Style Guide

Goals

Following this style guide should:

Make it easier to read and begin understanding unfamiliar code.
Make code easier to maintain.
Reduce simple programmer errors.
Reduce cognitive load while coding.
Keep discussions on diffs focused on the code's logic rather than its style.

Note that brevity is not a primary goal. Code should be made more concise only if other good code qualities (such as readability, simplicity, and clarity) remain equal or are improved.

Guiding Tenets

This guide is in addition to the official Swift API Design Guidelines. These rules should not contradict that document.
These rules should not fight Xcode's ^ + I indentation behavior.
We strive to make every rule lintable:
- If a rule changes the format of the code, it needs to be able to be reformatted automatically (either using SwiftFormat or SwiftLint autocorrect).
- For rules that don't directly change the format of the code, we should have a lint rule that throws a warning.
- Exceptions to these rules should be rare and heavily justified.

Swift Package Manager command plugin

This repo includes a Swift Package Manager command plugin that you can use to automatically reformat or lint your package according to the style guide. To use this command plugin with your package, all you need to do is add this repo as a dependency:

dependencies: [
  .package(url: "https://github.com/airbnb/swift", from: "1.0.0"),
]

and then run the format command plugin in your package directory:

$ swift package format

Usage guide

# Supported in Xcode 14+. Prompts for permission to write to the package directory.
$ swift package format

# When using the Xcode 13 toolchain, or a noninteractive shell, you must use: 
$ swift package --allow-writing-to-package-directory format

# To just lint without reformatting, you can use `--lint`:
$ swift package format --lint

# By default the command plugin runs on the entire package directory.
# You can exclude directories using `exclude`:
$ swift package format --exclude Tests

# Alternatively you can explicitly list the set of paths and/or SPM targets:
$ swift package format --paths Sources Tests Package.swift
$ swift package format --targets AirbnbSwiftFormatTool

# The plugin infers your package's minimum Swift version from the `swift-tools-version`
# in your `Package.swift`, but you can provide a custom value with `--swift-version`:
$ swift package format --swift-version 5.3

Xcode Formatting

You can enable the following settings in Xcode by running this script, e.g. as part of a "Run Script" build phase.

(link) Each line should have a maximum column width of 100 characters.

Why?

Due to larger screen sizes, we have opted to choose a page guide greater than 80.

We currently only "strictly enforce" (lint / auto-format) a maximum column width of 130 characters to limit the cases where manual clean up is required for reformatted lines that fall slightly above the threshold.
(link) Use 2 spaces to indent lines.
(link) Trim trailing whitespace in all lines.

⬆ back to top

Naming

(link) Use PascalCase for type and protocol names, and lowerCamelCase for everything else.

protocol SpaceThing {
  // ...
}

class SpaceFleet: SpaceThing {

  enum Formation {
    // ...
  }

  class Spaceship {
    // ...
  }

  var ships: [Spaceship] = []
  static let worldName: String = "Earth"

  func addShip(_ ship: Spaceship) {
    // ...
  }
}

let myFleet = SpaceFleet()

Exception: You may prefix a private property with an underscore if it is backing an identically-named property or method with a higher access level

Why?

There are specific scenarios where a backing a property or method could be easier to read than using a more descriptive name.

Type erasure

public final class AnyRequester<ModelType>: Requester {

  public init<T: Requester>(_ requester: T) where T.ModelType == ModelType {
    _executeRequest = requester.executeRequest
  }

  @discardableResult
  public func executeRequest(
    _ request: URLRequest,
    onSuccess: @escaping (ModelType, Bool) -> Void,
    onFailure: @escaping (Error) -> Void)
    -> URLSessionCancellable
  {
    return _executeRequest(request, session, parser, onSuccess, onFailure)
  }

  private let _executeRequest: (
    URLRequest,
    @escaping (ModelType, Bool) -> Void,
    @escaping (NSError) -> Void)
    -> URLSessionCancellable

}

Backing a less specific type with a more specific type

final class ExperiencesViewController: UIViewController {
  // We can't name this view since UIViewController has a view: UIView property.
  private lazy var _view = CustomView()

  loadView() {
    self.view = _view
  }
}

(link) Name booleans like isSpaceship, hasSpacesuit, etc. This makes it clear that they are booleans and not other types.

(link) Acronyms in names (e.g. URL) should be all-caps except when it’s the start of a name that would otherwise be lowerCamelCase, in which case it should be uniformly lower-cased.

// WRONG
class UrlValidator {

  func isValidUrl(_ URL: URL) -> Bool {
    // ...
  }

  func isProfileUrl(_ URL: URL, for userId: String) -> Bool {
    // ...
  }
}

let URLValidator = UrlValidator()
let isProfile = URLValidator.isProfileUrl(URLToTest, userId: IDOfUser)

// RIGHT
class URLValidator {

  func isValidURL(_ url: URL) -> Bool {
    // ...
  }

  func isProfileURL(_ url: URL, for userID: String) -> Bool {
    // ...
  }
}

let urlValidator = URLValidator()
let isProfile = urlValidator.isProfileURL(urlToTest, userID: idOfUser)

(link) Names should be written with their most general part first and their most specific part last. The meaning of "most general" depends on context, but should roughly mean "that which most helps you narrow down your search for the item you're looking for." Most importantly, be consistent with how you order the parts of your name.
```
// WRONG
let rightTitleMargin: CGFloat
let leftTitleMargin: CGFloat
let bodyRightMargin: CGFloat
let bodyLeftMargin: CGFloat

// RIGHT
let titleMarginRight: CGFloat
let titleMarginLeft: CGFloat
let bodyMarginRight: CGFloat
let bodyMarginLeft: CGFloat
```

(link) Include a hint about type in a name if it would otherwise be ambiguous.

// WRONG
let title: String
let cancel: UIButton

// RIGHT
let titleText: String
let cancelButton: UIButton

(link) Event-handling functions should be named like past-tense sentences. The subject can be omitted if it's not needed for clarity.

// WRONG
class ExperiencesViewController {

  private func handleBookButtonTap() {
    // ...
  }

  private func modelChanged() {
    // ...
  }
}

// RIGHT
class ExperiencesViewController {

  private func didTapBookButton() {
    // ...
  }

  private func modelDidChange() {
    // ...
  }
}

(link) Avoid Objective-C-style acronym prefixes. This is no longer needed to avoid naming conflicts in Swift.
```
// WRONG
class AIRAccount {
  // ...
}

// RIGHT
class Account {
  // ...
}
```
(link) Avoid *Controller in names of classes that aren't view controllers.

Why?

Controller is an overloaded suffix that doesn't provide information about the responsibilities of the class.

⬆ back to top

Style

(link) Don't include types where they can be easily inferred.

// WRONG
let host: Host = Host()

// RIGHT
let host = Host()

enum Direction {
  case left
  case right
}

func someDirection() -> Direction {
  // WRONG
  return Direction.left

  // RIGHT
  return .left
}

(link) Don't use self unless it's necessary for disambiguation or required by the language.

final class Listing {

  init(capacity: Int, allowsPets: Bool) {
    // WRONG
    self.capacity = capacity
    self.isFamilyFriendly = !allowsPets // `self.` not required here

    // RIGHT
    self.capacity = capacity
    isFamilyFriendly = !allowsPets
  }

  private let isFamilyFriendly: Bool
  private var capacity: Int

  private func increaseCapacity(by amount: Int) {
    // WRONG
    self.capacity += amount

    // RIGHT
    capacity += amount

    // WRONG
    self.save()

    // RIGHT
    save()
  }
}

(link) Bind to self when upgrading from a weak reference.

//WRONG
class MyClass {

  func request(completion: () -> Void) {
    API.request() { [weak self] response in
      guard let strongSelf = self else { return }
      // Do work
      completion()
    }
  }
}

// RIGHT
class MyClass {

  func request(completion: () -> Void) {
    API.request() { [weak self] response in
      guard let self else { return }
      // Do work
      completion()
    }
  }
}

(link) Add a trailing comma on the last element of a multi-line array.

// WRONG
let rowContent = [
  listingUrgencyDatesRowContent(),
  listingUrgencyBookedRowContent(),
  listingUrgencyBookedShortRowContent()
]

// RIGHT
let rowContent = [
  listingUrgencyDatesRowContent(),
  listingUrgencyBookedRowContent(),
  listingUrgencyBookedShortRowContent(),
]

(link) There should be no spaces inside the brackets of collection literals.

// WRONG
let innerPlanets = [ mercury, venus, earth, mars ]
let largestObjects = [ .star: sun, .planet: jupiter  ]

// RIGHT
let innerPlanets = [mercury, venus, earth, mars]
let largestObjects = [.star: sun, .planet: jupiter]

(link) Name members of tuples for extra clarity. Rule of thumb: if you've got more than 3 fields, you should probably be using a struct.

// WRONG
func whatever() -> (Int, Int) {
  return (4, 4)
}
let thing = whatever()
print(thing.0)

// RIGHT
func whatever() -> (x: Int, y: Int) {
  return (x: 4, y: 4)
}

// THIS IS ALSO OKAY
func whatever2() -> (x: Int, y: Int) {
  let x = 4
  let y = 4
  return (x, y)
}

let coord = whatever()
coord.x
coord.y

(link) Place the colon immediately after an identifier, followed by a space.

// WRONG
var something : Double = 0

// RIGHT
var something: Double = 0

// WRONG
class MyClass : SuperClass {
  // ...
}

// RIGHT
class MyClass: SuperClass {
  // ...
}

// WRONG
var dict = [KeyType:ValueType]()
var dict = [KeyType : ValueType]()

// RIGHT
var dict = [KeyType: ValueType]()

(link) Place a space on either side of a return arrow for readability.

// WRONG
func doSomething()->String {
  // ...
}

// RIGHT
func doSomething() -> String {
  // ...
}

// WRONG
func doSomething(completion: ()->Void) {
  // ...
}

// RIGHT
func doSomething(completion: () -> Void) {
  // ...
}

(link) Omit unnecessary parentheses.

// WRONG
if (userCount > 0) { ... }
switch (someValue) { ... }
let evens = userCounts.filter { (number) in number.isMultiple(of: 2) }
let squares = userCounts.map() { $0 * $0 }

// RIGHT
if userCount > 0 { ... }
switch someValue { ... }
let evens = userCounts.filter { number in number.isMultiple(of: 2) }
let squares = userCounts.map { $0 * $0 }

(link) Omit enum associated values from case statements when all arguments are unlabeled.

// WRONG
if case .done(_) = result { ... }

switch animal {
case .dog(_, _, _):
  ...
}

// RIGHT
if case .done = result { ... }

switch animal {
case .dog:
  ...
}

(link) When destructuring an enum case or a tuple, place the let keyword inline, adjacent to each individual property assignment.

// WRONG
switch result {
case let .success(value):
  // ...
case let .error(errorCode, errorReason):
  // ...
}

// WRONG
guard let case .success(value) else {
  return
}

// RIGHT
switch result {
case .success(let value):
  // ...
case .error(let errorCode, let errorReason):
  // ...
}

// RIGHT
guard case .success(let value) else {
  return
}

Why?

Consistency: We should prefer to either always inline the let keyword or never inline the let keyword. In Airbnb's Swift codebase, we observed that inline let is used far more often in practice (especially when destructuring enum cases with a single associated value).
Clarity: Inlining the let keyword makes it more clear which identifiers are part of the conditional check and which identifiers are binding new variables, since the let keyword is always adjacent to the variable identifier.

// `let` is adjacent to the variable identifier, so it is immediately obvious
// at a glance that these identifiers represent new variable bindings
case .enumCaseWithSingleAssociatedValue(let string):
case .enumCaseWithMultipleAssociatedValues(let string, let int):

// The `let` keyword is quite far from the variable identifiers,
// so it is less obvious that they represent new variable bindings
case let .enumCaseWithSingleAssociatedValue(string):
case let .enumCaseWithMultipleAssociatedValues(string, int):

(link) Place function/type attributes on the line above the declaration.

// WRONG
@objc class Spaceship {

  @discardableResult func fly() -> Bool {
  }
}

// RIGHT

@objc
class Spaceship {

  @discardableResult
  func fly() -> Bool {
  }
}

(link) Multi-line arrays should have each bracket on a separate line. Put the opening and closing brackets on separate lines from any of the elements of the array. Also add a trailing comma on the last element.

// WRONG
let rowContent = [listingUrgencyDatesRowContent(),
                  listingUrgencyBookedRowContent(),
                  listingUrgencyBookedShortRowContent()]

let rowContent = [
  listingUrgencyDatesRowContent(),
  listingUrgencyBookedRowContent(),
  listingUrgencyBookedShortRowContent()
]

// RIGHT
let rowContent = [
  listingUrgencyDatesRowContent(),
  listingUrgencyBookedRowContent(),
  listingUrgencyBookedShortRowContent(),
]

(link) Long typealiases of protocol compositions should wrap before the = and before each individual &.

// WRONG (too long)
public typealias Dependencies = UniverseBuilderProviding & LawsOfPhysicsProviding & UniverseSimulatorServiceProviding & PlanetBuilderProviding & CivilizationServiceProviding

// WRONG (naive wrapping)
public typealias Dependencies = UniverseBuilderProviding & LawsOfPhysicsProviding & UniverseSimulatorServiceProviding &
  PlanetBuilderProviding & CivilizationServiceProviding

// WRONG (unbalanced)
public typealias Dependencies = UniverseBuilderProviding
  & LawsOfPhysicsProviding
  & UniverseSimulatorServiceProviding
  & PlanetBuilderProviding
  & CivilizationServiceProviding

// RIGHT
public typealias Dependencies
  = UniverseBuilderProviding
  & LawsOfPhysicsProviding
  & UniverseSimulatorServiceProviding
  & PlanetBuilderProviding
  & CivilizationServiceProviding

(link) Omit the right-hand side of the expression when unwrapping an optional property to a non-optional property with the same name.

Why?

Following the rationale in SE-0345, this shorthand syntax removes unnecessary boilerplate while retaining clarity.

// WRONG
if
  let galaxy = galaxy,
  galaxy.name == "Milky Way"
{ … }

guard
  let galaxy = galaxy,
  galaxy.name == "Milky Way"
else { … }

// RIGHT
if
  let galaxy,
  galaxy.name == "Milky Way"
{ … }

guard
  let galaxy,
  galaxy.name == "Milky Way"
else { … }

(link) Multi-line conditional statements should break after the leading keyword. Indent each individual statement by 2 spaces.

Why?

Breaking after the leading keyword resets indentation to the standard 2-space grid, which helps avoid fighting Xcode's ^ + I indentation behavior.

// WRONG
if let galaxy,
  galaxy.name == "Milky Way" // Indenting by two spaces fights Xcode's ^+I indentation behavior
{ … }

// WRONG
guard let galaxy,
      galaxy.name == "Milky Way" // Variable width indentation (6 spaces)
else { … }

// WRONG
guard let earth = universe.find(
  .planet,
  named: "Earth"),
  earth.isHabitable // Blends in with previous condition's method arguments
else { … }

// RIGHT
if
  let galaxy,
  galaxy.name == "Milky Way"
{ … }

// RIGHT
guard
  let galaxy,
  galaxy.name == "Milky Way"
else { … }

// RIGHT
guard
  let earth = universe.find(
    .planet,
    named: "Earth"),
  earth.isHabitable
else { … }

// RIGHT
if let galaxy {
  …
}

// RIGHT
guard let galaxy else {
  …
}

(link) Indent the body and closing triple-quote of multiline string literals, unless the string literal begins on its own line in which case the string literal contents and closing triple-quote should have the same indentation as the opening triple-quote.

// WRONG
var spaceQuote = """
“Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is.
I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”
"""

// RIGHT
var spaceQuote = """
  “Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is.
  I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”
  """

// WRONG
var universeQuote: String {
  """
    In the beginning the Universe was created.
    This has made a lot of people very angry and been widely regarded as a bad move.
    """
}

// RIGHT
var universeQuote: String {
  """
  In the beginning the Universe was created.
  This has made a lot of people very angry and been widely regarded as a bad move.
  """
}

(link) Use constructors instead of Make() functions for NSRange and others.

// WRONG
let range = NSMakeRange(10, 5)

// RIGHT
let range = NSRange(location: 10, length: 5)

(link) For standard library types with a canonical shorthand form (Optional, Array, Dictionary), prefer using the shorthand form over the full generic form.

// WRONG
let optional: Optional<String> = nil
let array: Array<String> = []
let dictionary: Dictionary<String, Any> = [:]

// RIGHT
let optional: String? = nil
let array: [String] = []
let dictionary: [String: Any] = [:]

(link) Omit explicit .init when not required.

// WRONG
let universe = Universe.init()

// RIGHT
let universe = Universe()

(link) The opening brace following a single-line expression should be on the same line as the rest of the statement.

// WRONG
if !planet.isHabitable
{
  planet.terraform()
}

class Planet
{
  func terraform()
  {
    generateAtmosphere()
    generateOceans()
  }
}

// RIGHT
if !planet.isHabitable {
  planet.terraform()
}

class Planet {
  func terraform() {
    generateAtmosphere()
    generateOceans()
  }
}

(link) The opening brace following a multi-line expression should wrap to a new line.

// WRONG
if
  let star = planet.nearestStar(),
  planet.isInHabitableZone(of: star) {
  planet.terraform()
}

class Planet {
  func terraform(
    atmosphereOptions: AtmosphereOptions = .default,
    oceanOptions: OceanOptions = .default) {
    generateAtmosphere(atmosphereOptions)
    generateOceans(oceanOptions)
  }
}

// RIGHT
if
  let star = planet.nearestStar(),
  planet.isInHabitableZone(of: star)
{
  planet.terraform()
}

class Planet {
  func terraform(
    atmosphereOptions: AtmosphereOptions = .default,
    oceanOptions: OceanOptions = .default) 
  {
    generateAtmosphere(atmosphereOptions)
    generateOceans(oceanOptions)
  }
}

(link) Braces should be surrounded by a single whitespace character (either a space, or a newline) on each side.

// WRONG
struct Planet{
  …
}

// WRONG
if condition{
  …
}else{
  …
}

// RIGHT
struct Planet {
  …
}

// RIGHT
if condition {
  …
} else {
  …
}

(link) For function calls and declarations, there should be no spaces before or inside the parentheses of the argument list.

// WRONG
func install ( _ engine: Engine ) { }

install ( AntimatterDrive( ) )

// RIGHT
func install(_ engine: Engine) { }

install(AntimatterDrive())

(link) Comment blocks should use single-line comments (// for code comments and /// for documentation comments), rather than multi-line comments (/* ... */ and /** ... */).

// WRONG

/**
* A planet that exists somewhere in the universe.
*
* Planets have many properties. For example, the best planets
* have atmospheres and bodies of water to support life.
*/
class Planet {
  /**
    Terraforms the planet, by adding an atmosphere and ocean that is hospitable for life.
  */
  func terraform() {
    /* 
    Generate the atmosphere first, before generating the ocean.
    Otherwise, the water will just boil off immediately.
    */
    generateAtmosphere()

    /* Now that we have an atmosphere, it's safe to generate the ocean */
    generateOceans()
  }
}

// RIGHT

/// A planet that exists somewhere in the universe.
///
/// Planets have many properties. For example, the best planets
/// have atmospheres and bodies of water to support life.
class Planet {
  /// Terraforms the planet, by adding an atmosphere and ocean that is hospitable for life.
  func terraform() {
    // Generate the atmosphere first, before generating the ocean.
    // Otherwise, the water will just boil off immediately.
    generateAtmosphere()

    // Now that we have an atmosphere, it's safe to generate the ocean
    generateOceans()
  }
}

(link) Include spaces or newlines before and after comment delimiters (//, ///, /*, and */)

// WRONG

///A spacecraft with incredible performance characteristics
struct Spaceship {

  func travelFasterThanLight() {/*unimplemented*/}

  func travelBackInTime() { }//TODO: research whether or not this is possible

}

// RIGHT

/// A spacecraft with incredible performance characteristics
struct Spaceship {

  func travelFasterThanLight() { /* unimplemented */ }

  func travelBackInTime() { } // TODO: research whether or not this is possible

}

(link) Include a single space in an empty set of braces ({ }).

// WRONG
extension Spaceship: Trackable {}

extension SpaceshipView {
  var accessibilityIdentifier: String {
    get { spaceship.name }
    set {}
  }
}

// RIGHT
extension Spaceship: Trackable { }

extension SpaceshipView {
  var accessibilityIdentifier: String {
    get { spaceship.name }
    set { }
  }
}

Functions

(link) Omit Void return types from function definitions.

// WRONG
func doSomething() -> Void {
  ...
}

// RIGHT
func doSomething() {
  ...
}

(link) Separate long function declarations with line breaks before each argument label and before the return signature. Put the open curly brace on the next line so the first executable line doesn't look like it's another parameter.

class Universe {

  // WRONG
  func generateStars(at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) -> String {
    // This is too long and will probably auto-wrap in a weird way
  }

  // WRONG
  func generateStars(at location: Point,
                     count: Int,
                     color: StarColor,
                     withAverageDistance averageDistance: Float) -> String
  {
    // Xcode indents all the arguments
  }

  // WRONG
  func generateStars(
    at location: Point,
    count: Int,
    color: StarColor,
    withAverageDistance averageDistance: Float) -> String {
    populateUniverse() // this line blends in with the argument list
  }

  // WRONG
  func generateStars(
    at location: Point,
    count: Int,
    color: StarColor,
    withAverageDistance averageDistance: Float) throws
    -> String {
    populateUniverse() // this line blends in with the argument list
  }

  // RIGHT
  func generateStars(
    at location: Point,
    count: Int,
    color: StarColor,
    withAverageDistance averageDistance: Float)
    -> String
  {
    populateUniverse()
  }

  // RIGHT
  func generateStars(
    at location: Point,
    count: Int,
    color: StarColor,
    withAverageDistance averageDistance: Float)
    throws -> String
  {
    populateUniverse()
  }
}

(link) Long function invocations should also break on each argument. Put the closing parenthesis on the last line of the invocation.

// WRONG
universe.generateStars(at: location, count: 5, color: starColor, withAverageDistance: 4)

// WRONG
universe.generateStars(at: location,
                       count: 5,
                       color: starColor,
                       withAverageDistance: 4)

// WRONG
universe.generateStars(
  at: location,
  count: 5,
  color: starColor,
  withAverageDistance: 4
)

// WRONG
universe.generate(5,
  .stars,
  at: location)

// RIGHT
universe.generateStars(
  at: location,
  count: 5,
  color: starColor,
  withAverageDistance: 4)

// RIGHT
universe.generate(
  5,
  .stars,
  at: location)

(link) Name unused function parameters as underscores (_).

Why?

Naming unused function parameters as underscores makes it more clear when the parameter is unused within the function body. This can make it easier to catch subtle logical errors, and can highlight opportunities to simplify method signatures.

// WRONG

// In this method, the `newContext` parameter is unused.
// This is actually a logical error, and is easy to miss, but compiles without warning.
func withContext(_ newContext: Context) {
  var updatedValue = self
  updatedValue.context = context
  return updatedValue  
}

// In this method, the `color` parameter is unused.
// Is this a logical error (e.g. should it be passed through to the `universe.generateStars` method call),
// or is this an unused argument that should be removed from the method signature?
func generateUniverseWithStars(
  at location: Point,
  count: Int,
  color: StarColor,
  withAverageDistance averageDistance: Float)
{
  let universe = generateUniverse()
  universe.generateStars(
    at: location,
    count: count,
    withAverageDistance: averageDistance)
}

// RIGHT

// Automatically reformatting the unused parameter to be an underscore
// makes it more clear that the parameter is unused, which makes it
// easier to spot the logical error.
func withContext(_: Context) {
  var updatedValue = self
  updatedValue.context = context
  return updatedValue  
}

// The underscore makes it more clear that the `color` parameter is unused.
// This method argument can either be removed if truly unnecessary,
// or passed through to `universe.generateStars` to correct the logical error.
func generateUniverseWithStars(
  at location: Point,
  count: Int,
  color _: StarColor,
  withAverageDistance averageDistance: Float)
{
  let universe = generateUniverse()
  universe.generateStars(
    at: location,
    count: count,
    withAverageDistance: averageDistance)
}

Closures

(link) Favor Void return types over () in closure declarations. If you must specify a Void return type in a function declaration, use Void rather than () to improve readability.
```
// WRONG
func method(completion: () -> ()) {
  ...
}

// RIGHT
func method(completion: () -> Void) {
  ...
}
```
(link) Name unused closure parameters as underscores (_).
Why?

Naming unused closure parameters as underscores reduces the cognitive overhead required to read closures by making it obvious which parameters are used and which are unused.
```
// WRONG
someAsyncThing() { argument1, argument2, argument3 in
  print(argument3)
}

// RIGHT
someAsyncThing() { _, _, argument3 in
  print(argument3)
}
```

(link) Closures should have a single space or newline inside each brace. Trailing closures should additionally have a single space or newline outside each brace.

// WRONG
let evenSquares = numbers.filter{$0.isMultiple(of: 2)}.map{  $0 * $0  }

// RIGHT
let evenSquares = numbers.filter { $0.isMultiple(of: 2) }.map { $0 * $0 }

// WRONG
let evenSquares = numbers.filter( { $0.isMultiple(of: 2) } ).map( { $0 * $0 } )

// RIGHT
let evenSquares = numbers.filter({ $0.isMultiple(of: 2) }).map({ $0 * $0 })

// WRONG
let evenSquares = numbers
  .filter{ 
    $0.isMultiple(of: 2) 
  }
  .map{ 
    $0 * $0 
  }

// RIGHT
let evenSquares = numbers
  .filter {
    $0.isMultiple(of: 2) 
  }
  .map {
    $0 * $0 
  }

(link) Omit Void return types from closure expressions.

// WRONG
someAsyncThing() { argument -> Void in
  ...
}

// RIGHT
someAsyncThing() { argument in
  ...
}

(link) Prefer trailing closure syntax for closure arguments with no parameter name.

// WRONG
planets.map({ $0.name })

// RIGHT
planets.map { $0.name }

// ALSO RIGHT, since this closure has a parameter name
planets.first(where: { $0.isGasGiant })

// ALSO FINE. Trailing closure syntax is still permitted for closures
// with parameter names. However, consider using non-trailing syntax
// in cases where the parameter name is semantically meaningful.
planets.first { $0.isGasGiant }

Operators

(link) Infix operators should have a single space on either side. Prefer parenthesis to visually group statements with many operators rather than varying widths of whitespace. This rule does not apply to range operators (e.g. 1...3) and postfix or prefix operators (e.g. guest? or -1).

// WRONG
let capacity = 1+2
let capacity = currentCapacity   ?? 0
let mask = (UIAccessibilityTraitButton|UIAccessibilityTraitSelected)
let capacity=newCapacity
let latitude = region.center.latitude - region.span.latitudeDelta/2.0

// RIGHT
let capacity = 1 + 2
let capacity = currentCapacity ?? 0
let mask = (UIAccessibilityTraitButton | UIAccessibilityTraitSelected)
let capacity = newCapacity
let latitude = region.center.latitude - (region.span.latitudeDelta / 2.0)

(link) Long ternary operator expressions should wrap before the ? and before the :, putting each conditional branch on a separate line.

// WRONG (too long)
let destinationPlanet = solarSystem.hasPlanetsInHabitableZone ? solarSystem.planetsInHabitableZone.first : solarSystem.uninhabitablePlanets.first

// WRONG (naive wrapping)
let destinationPlanet = solarSystem.hasPlanetsInHabitableZone ? solarSystem.planetsInHabitableZone.first :
  solarSystem.uninhabitablePlanets.first

// WRONG (unbalanced operators)
let destinationPlanet = solarSystem.hasPlanetsInHabitableZone ?
  solarSystem.planetsInHabitableZone.first :
  solarSystem.uninhabitablePlanets.first

// RIGHT
let destinationPlanet = solarSystem.hasPlanetsInHabitableZone
  ? solarSystem.planetsInHabitableZone.first
  : solarSystem.uninhabitablePlanets.first

(link) In conditional statements (if, guard, while), separate boolean conditions using commas (,) instead of && operators.

// WRONG
if let star = planet.star, !planet.isHabitable && planet.isInHabitableZone(of: star) {
  planet.terraform()
}

if
  let star = planet.star, 
  !planet.isHabitable 
  && planet.isInHabitableZone(of: star)
{
  planet.terraform()
}

// RIGHT
if let star = planet.star, !planet.isHabitable, planet.isInHabitableZone(of: star) {
  planet.terraform()
}

if
  let star = planet.star,
  !planet.isHabitable,
  planet.isInHabitableZone(of: star)
{
  planet.terraform()
}

(link) When extending bound generic types, prefer using generic bracket syntax (extension Collection<Planet>), or sugared syntax for applicable standard library types (extension [Planet]) instead of generic type constraints.

// WRONG
extension Array where Element == Star { … }
extension Optional where Wrapped == Spaceship { … }
extension Dictionary where Key == Moon, Element == Planet { … }
extension Collection where Element == Universe { … }
extension StateStore where State == SpaceshipState, Action == SpaceshipAction { … }

// RIGHT
extension [Star] { … }
extension Spaceship? { … }
extension [Moon: Planet] { … }
extension Collection<Universe> { … }
extension StateStore<SpaceshipState, SpaceshipAction> { … }

// ALSO RIGHT -- there are multiple types that could satifsy this constraint
// (e.g. [Planet], [Moon]), so this is not a "bound generic type" and isn't
// eligible for the generic bracket syntax.
extension Array where Element: PlanetaryBody { }

⬆ back to top

Patterns

(link) Prefer initializing properties at init time whenever possible, rather than using implicitly unwrapped optionals. A notable exception is UIViewController's view property.

// WRONG
class MyClass {

  init() {
    super.init()
    someValue = 5
  }

  var someValue: Int!
}

// RIGHT
class MyClass {

  init() {
    someValue = 0
    super.init()
  }

  var someValue: Int
}

(link) Avoid performing any meaningful or time-intensive work in init(). Avoid doing things like opening database connections, making network requests, reading large amounts of data from disk, etc. Create something like a start() method if these things need to be done before an object is ready for use.

(link) Extract complex property observers into methods. This reduces nestedness, separates side-effects from property declarations, and makes the usage of implicitly-passed parameters like oldValue explicit.

// WRONG
class TextField {
  var text: String? {
    didSet {
      guard oldValue != text else {
        return
      }

      // Do a bunch of text-related side-effects.
    }
  }
}

// RIGHT
class TextField {
  var text: String? {
    didSet { textDidUpdate(from: oldValue) }
  }

  private func textDidUpdate(from oldValue: String?) {
    guard oldValue != text else {
      return
    }

    // Do a bunch of text-related side-effects.
  }
}

(link) Extract complex callback blocks into methods. This limits the complexity introduced by weak-self in blocks and reduces nestedness. If you need to reference self in the method call, make use of guard to unwrap self for the duration of the callback.

//WRONG
class MyClass {

  func request(completion: () -> Void) {
    API.request() { [weak self] response in
      if let self {
        // Processing and side effects
      }
      completion()
    }
  }
}

// RIGHT
class MyClass {

  func request(completion: () -> Void) {
    API.request() { [weak self] response in
      guard let self else { return }
      self.doSomething(with: self.property, response: response)
      completion()
    }
  }

  func doSomething(with nonOptionalParameter: SomeClass, response: SomeResponseClass) {
    // Processing and side effects
  }
}

(link) Prefer using guard at the beginning of a scope.

Why?

It's easier to reason about a block of code when all guard statements are grouped together at the top rather than intermixed with business logic.

(link) Access control should be at the strictest level possible. Prefer public to open and private to fileprivate unless you need that behavior.

// WRONG
public struct Spaceship {
  // WRONG: `engine` is used in `extension Spaceship` below,
  // but extensions in the same file can access `private` members.
  fileprivate let engine: AntimatterEngine

  // WRONG: `hull` is not used by any other type, so `fileprivate` is unnecessary. 
  fileprivate let hull: Hull

  // RIGHT: `navigation` is used in `extension Pilot` below,
  // so `fileprivate` is necessary here.
  fileprivate let navigation: SpecialRelativityNavigationService
}

extension Spaceship {
  public func blastOff() {
    engine.start()
  }
}

extension Pilot {
  public func chartCourse() {
    spaceship.navigation.course = .andromedaGalaxy
    spaceship.blastOff()
  }
}

// RIGHT
public struct Spaceship {
  fileprivate let navigation: SpecialRelativityNavigationService
  private let engine: AntimatterEngine
  private let hull: Hull
}

extension Spaceship {
  public func blastOff() {
    engine.start()
  }
}

extension Pilot {
  public func chartCourse() {
    spaceship.navigation.course = .andromedaGalaxy
    spaceship.blastOff()
  }
}

(link) Avoid global functions whenever possible. Prefer methods within type definitions.

// WRONG
func age(of person: Person, bornAt: TimeInterval) -> Int {
  // ...
}

func jump(person: Person) {
  // ...
}

// RIGHT
class Person {
  var bornAt: TimeInterval

  var age: Int {
    // ...
  }

  func jump() {
    // ...
  }
}

(link) Use caseless enums for organizing public or internal constants and functions into namespaces.
- Avoid creating non-namespaced global constants and functions.
- Feel free to nest namespaces where it adds clarity.
- private globals are permitted, since they are scoped to a single file and do not pollute the global namespace. Consider placing private globals in an enum namespace to match the guidelines for other declaration types.
Why?

Caseless enums work well as namespaces because they cannot be instantiated, which matches their intent.
```
enum Environment {

  enum Earth {
    static let gravity = 9.8
  }

  enum Moon {
    static let gravity = 1.6
  }
}
```

(link) Use Swift's automatic enum values unless they map to an external source. Add a comment explaining why explicit values are defined.

Why?

To minimize user error, improve readability, and write code faster, rely on Swift's automatic enum values. If the value maps to an external source (e.g. it's coming from a network request) or is persisted across binaries, however, define the values explicitly, and document what these values are mapping to.

This ensures that if someone adds a new value in the middle, they won't accidentally break things.

// WRONG
enum ErrorType: String {
  case error = "error"
  case warning = "warning"
}

enum UserType: String {
  case owner
  case manager
  case member
}

enum Planet: Int {
  case mercury = 0
  case venus = 1
  case earth = 2
  case mars = 3
  case jupiter = 4
  case saturn = 5
  case uranus = 6
  case neptune = 7
}

enum ErrorCode: Int {
  case notEnoughMemory
  case invalidResource
  case timeOut
}

// RIGHT
enum ErrorType: String {
  case error
  case warning
}

/// These are written to a logging service. Explicit values ensure they're consistent across binaries.
// swiftformat:disable redundantRawValues
enum UserType: String {
  case owner = "owner"
  case manager = "manager"
  case member = "member"
}
// swiftformat:enable redundantRawValues

enum Planet: Int {
  case mercury
  case venus
  case earth
  case mars
  case jupiter
  case saturn
  case uranus
  case neptune
}

/// These values come from the server, so we set them here explicitly to match those values.
enum ErrorCode: Int {
  case notEnoughMemory = 0
  case invalidResource = 1
  case timeOut = 2
}

(link) Use optionals only when they have semantic meaning.

(link) Prefer immutable values whenever possible. Use map and compactMap instead of appending to a new collection. Use filter instead of removing elements from a mutable collection.

Why?

Mutable variables increase complexity, so try to keep them in as narrow a scope as possible.

// WRONG
var results = [SomeType]()
for element in input {
  let result = transform(element)
  results.append(result)
}

// RIGHT
let results = input.map { transform($0) }

// WRONG
var results = [SomeType]()
for element in input {
  if let result = transformThatReturnsAnOptional(element) {
    results.append(result)
  }
}

// RIGHT
let results = input.compactMap { transformThatReturnsAnOptional($0) }

(link) Prefer immutable or computed static properties over mutable ones whenever possible. Use stored static let properties or computed static var properties over stored static vars properties whenever possible, as stored static var properties are global mutable state.

Why?

Global mutable state increases complexity and makes it harder to reason about the behavior of applications. It should be avoided when possible.

// WRONG
enum Fonts {
  static var title = UIFont(…)
}

// RIGHT
enum Fonts {
  static let title = UIFont(…)
}

// WRONG
struct FeatureState {
  var count: Int

  static var initial = FeatureState(count: 0)
}

// RIGHT
struct FeatureState {
  var count: Int

  static var initial: FeatureState {
    // Vend static properties that are cheap to compute
    FeatureState(count: 0)
  }
}

(link) Handle an unexpected but recoverable condition with an assert method combined with the appropriate logging in production. If the unexpected condition is not recoverable, prefer a precondition method or fatalError(). This strikes a balance between crashing and providing insight into unexpected conditions in the wild. Only prefer fatalError over a precondition method when the failure message is dynamic, since a precondition method won't report the message in the crash report.

func didSubmitText(_ text: String) {
  // It's unclear how this was called with an empty string; our custom text field shouldn't allow this.
  // This assert is useful for debugging but it's OK if we simply ignore this scenario in production.
  guard !text.isEmpty else {
    assertionFailure("Unexpected empty string")
    return
  }
  // ...
}

func transformedItem(atIndex index: Int, from items: [Item]) -> Item {
  precondition(index >= 0 && index < items.count)
  // It's impossible to continue executing if the precondition has failed.
  // ...
}

func makeImage(name: String) -> UIImage {
  guard let image = UIImage(named: name, in: nil, compatibleWith: nil) else {
    fatalError("Image named \(name) couldn't be loaded.")
    // We want the error message so we know the name of the missing image.
  }
  return image
}

(link) Default type methods to static.

Why?

If a method needs to be overridden, the author should opt into that functionality by using the class keyword instead.

// WRONG
class Fruit {
  class func eatFruits(_ fruits: [Fruit]) { ... }
}

// RIGHT
class Fruit {
  static func eatFruits(_ fruits: [Fruit]) { ... }
}

(link) Default classes to final.
Why?

If a class needs to be overridden, the author should opt into that functionality by omitting the final keyword.
```
// WRONG
class SettingsRepository {
  // ...
}

// RIGHT
final class SettingsRepository {
  // ...
}
```

(link) Never use the default case when switching over an enum.

Why?

Enumerating every case requires developers and reviewers have to consider the correctness of every switch statement when new cases are added.

// WRONG
switch anEnum {
case .a:
  // Do something
default:
  // Do something else.
}

// RIGHT
switch anEnum {
case .a:
  // Do something
case .b, .c:
  // Do something else.
}

(link) Check for nil rather than using optional binding if you don't need to use the value.
Why?

Checking for nil makes it immediately clear what the intent of the statement is. Optional binding is less explicit.
```
var thing: Thing?

// WRONG
if let _ = thing {
  doThing()
}

// RIGHT
if thing != nil {
  doThing()
}
```

(link) Omit the return keyword when not required by the language.

// WRONG
["1", "2", "3"].compactMap { return Int($0) }

var size: CGSize {
  return CGSize(
    width: 100.0,
    height: 100.0)
}

func makeInfoAlert(message: String) -> UIAlertController {
  return UIAlertController(
    title: "ℹ️ Info",
    message: message,
    preferredStyle: .alert)
}

// RIGHT
["1", "2", "3"].compactMap { Int($0) }

var size: CGSize {
  CGSize(
    width: 100.0,
    height: 100.0)
}

func makeInfoAlert(message: String) -> UIAlertController {
  UIAlertController(
    title: "ℹ️ Info",
    message: message,
    preferredStyle: .alert)
}

(link) Use AnyObject instead of class in protocol definitions.
Why?

SE-0156, which introduced support for using the AnyObject keyword as a protocol constraint, recommends preferring AnyObject over class:

This proposal merges the concepts of class and AnyObject, which now have the same meaning: they represent an existential for classes. To get rid of the duplication, we suggest only keeping AnyObject around. To reduce source-breakage to a minimum, class could be redefined as typealias class = AnyObject and give a deprecation warning on class for the first version of Swift this proposal is implemented in. Later, class could be removed in a subsequent version of Swift.
```
// WRONG
protocol Foo: class { }

// RIGHT
protocol Foo: AnyObject { }
```

(link) Specify the access control for each declaration in an extension individually.

Why?

Specifying the access control on the declaration itself helps engineers more quickly determine the access control level of an individual declaration.

// WRONG
public extension Universe {
  // This declaration doesn't have an explicit access control level.
  // In all other scopes, this would be an internal function,
  // but because this is in a public extension, it's actually a public function.
  func generateGalaxy() { }
}

// WRONG
private extension Spaceship {
  func enableHyperdrive() { }
}

// RIGHT
extension Universe {
  // It is immediately obvious that this is a public function,
  // even if the start of the `extension Universe` scope is off-screen.
  public func generateGalaxy() { }
}

// RIGHT
extension Spaceship {
  // Recall that a private extension actually has fileprivate semantics,
  // so a declaration in a private extension is fileprivate by default.
  fileprivate func enableHyperdrive() { }
}

(link) Prefer dedicated logging systems like os_log or swift-log over writing directly to standard out using print(…), debugPrint(…), or dump(…).

Why?

All log messages should flow into intermediate logging systems that can direct messages to the correct destination(s) and potentially filter messages based on the app's environment or configuration. print(…), debugPrint(…), or dump(…) will write all messages directly to standard out in all app configurations and can potentially leak personally identifiable information (PII).

(link) Avoid single-expression closures that are always called immediately. Instead, prefer inlining the expression.

// WRONG
lazy var universe: Universe = { 
  Universe() 
}()

lazy var stars = {
  universe.generateStars(
    at: location,
    count: 5,
    color: starColor,
    withAverageDistance: 4)
}()

// RIGHT
lazy var universe = Universe() 

lazy var stars = universe.generateStars(
  at: location,
  count: 5,
  color: starColor,
  withAverageDistance: 4)

(link) Omit the get clause from a computed property declaration that doesn't also have a set, willSet, or didSet clause.

// WRONG
var universe: Universe {
  get {
    Universe()
  }
}

// RIGHT
var universe: Universe {
  Universe()
}

// RIGHT
var universe: Universe {
  get { multiverseService.current }
  set { multiverseService.current = newValue }
}

⬆ back to top

File Organization

(link) Alphabetize and deduplicate module imports within a file. Place all imports at the top of the file below the header comments. Do not add additional line breaks between import statements. Add a single empty line before the first import and after the last import.

Why?

A standard organization method helps engineers more quickly determine which modules a file depends on.
Duplicated import statements have no effect and should be removed for clarity.

// WRONG

//  Copyright © 2018 Airbnb. All rights reserved.
//
import DLSPrimitives
import Constellation
import Constellation
import Epoxy

import Foundation

//RIGHT

//  Copyright © 2018 Airbnb. All rights reserved.
//

import Constellation
import DLSPrimitives
import Epoxy
import Foundation

Exception: @testable import should be grouped after the regular import and separated by an empty line.

// WRONG

//  Copyright © 2018 Airbnb. All rights reserved.
//

import DLSPrimitives
@testable import Epoxy
import Foundation
import Nimble
import Quick

//RIGHT

//  Copyright © 2018 Airbnb. All rights reserved.
//

import DLSPrimitives
import Foundation
import Nimble
import Quick

@testable import Epoxy

(link) Limit consecutive whitespace to one blank line or space (excluding indentation). Favor the following formatting guidelines over whitespace of varying heights or widths.

// WRONG
struct Planet {

  let mass:          Double
  let hasAtmosphere: Bool


  func distance(to: Planet) { }

}

// RIGHT
struct Planet {

  let mass: Double
  let hasAtmosphere: Bool

  func distance(to: Planet) { }

}

(link) Files should end in a newline.

(link) Declarations that include scopes spanning multiple lines should be separated from adjacent declarations in the same scope by a newline. Insert a single blank line between multi-line scoped declarations (e.g. types, extensions, functions, computed properties, etc.) and other declarations at the same indentation level.

Why?

Dividing scoped declarations from other declarations at the same scope visually separates them, making adjacent declarations easier to differentiate from the scoped declaration.

// WRONG
struct SolarSystem {
  var numberOfPlanets: Int {
    …
  }
  func distance(to: SolarSystem) -> AstronomicalUnit {
    …
  }
}
struct Galaxy {
  func distance(to: Galaxy) -> AstronomicalUnit {
    …
  }
  func contains(_ solarSystem: SolarSystem) -> Bool {
    …
  }
}

// RIGHT
struct SolarSystem {
  var numberOfPlanets: Int {
    …
  }

  func distance(to: SolarSystem) -> AstronomicalUnit {
    …
  }
}

struct Galaxy {
  func distance(to: Galaxy) -> AstronomicalUnit {
    …
  }

  func contains(_ solarSystem: SolarSystem) -> Bool {
    …
  }
}

(link) Remove blank lines at the top and bottom of scopes, excluding type bodies which can optionally include blank lines.

// WRONG
class Planet {
  func terraform() {

    generateAtmosphere()
    generateOceans()

  }
}

// RIGHT
class Planet {
  func terraform() {
    generateAtmosphere()
    generateOceans()
  } 
}

// Also fine!
class Planet {

  func terraform() {
    generateAtmosphere()
    generateOceans()
  }

}

(link) Each type and extension which implements a conformance should be preceded by a MARK comment.
- Types should be preceded by a // MARK: - TypeName comment.
- Extensions that add a conformance should be preceded by a // MARK: - TypeName + ProtocolName comment.
- Extensions that immediately follow the type being extended should omit that type's name and instead use // MARK: ProtocolName.
- If there is only one type or extension in a file, the MARK comment can be omitted.
- If the extension in question is empty (e.g. has no declarations in its body), the MARK comment can be omitted.
- For extensions that do not add new conformances, consider adding a MARK with a descriptive comment.
```
// MARK: - GalaxyView

final class GalaxyView: UIView { … }

// MARK: ContentConfigurableView

extension GalaxyView: ContentConfigurableView { … }

// MARK: - Galaxy + SpaceThing, NamedObject

extension Galaxy: SpaceThing, NamedObject { … }
```
(link) Use // MARK: to separate the contents of type definitions and extensions into the sections listed below, in order. All type definitions and extensions should be divided up in this consistent way, allowing a reader of your code to easily jump to what they are interested in.
- // MARK: Lifecycle for init and deinit methods.
- // MARK: Open for open properties and methods.
- // MARK: Public for public properties and methods.
- // MARK: Internal for internal properties and methods.
- // MARK: Fileprivate for fileprivate properties and methods.
- // MARK: Private for private properties and methods.
- If the type in question is an enum, its cases should go above the first // MARK:.
- Do not subdivide each of these sections into subsections, as it makes the method dropdown more cluttered and therefore less useful. Instead, group methods by functionality and use smart naming to make clear which methods are related. If there are enough methods that sub-sections seem necessary, consider refactoring your code into multiple types.
- If all of the type or extension's definitions belong to the same category (e.g. the type or extension only consists of internal properties), it is OK to omit the // MARK:s.
- If the type in question is a simple value type (e.g. fewer than 20 lines), it is OK to omit the // MARK:s, as it would hurt legibility.
(link) Within each top-level section, place content in the following order. This allows a new reader of your code to more easily find what they are looking for.
- Nested types and typealiases
- Static properties
- Class properties
- Instance properties
- Static methods
- Class methods
- Instance methods

(link) Add empty lines between property declarations of different kinds. (e.g. between static properties and instance properties.)

// WRONG
static let gravityEarth: CGFloat = 9.8
static let gravityMoon: CGFloat = 1.6
var gravity: CGFloat

// RIGHT
static let gravityEarth: CGFloat = 9.8
static let gravityMoon: CGFloat = 1.6

var gravity: CGFloat

(link) Computed properties and properties with property observers should appear at the end of the set of declarations of the same kind. (e.g. instance properties.)

// WRONG
var atmosphere: Atmosphere {
  didSet {
    print("oh my god, the atmosphere changed")
  }
}
var gravity: CGFloat

// RIGHT
var gravity: CGFloat
var atmosphere: Atmosphere {
  didSet {
    print("oh my god, the atmosphere changed")
  }
}

⬆ back to top

Objective-C Interoperability

(link) Prefer pure Swift classes over subclasses of NSObject. If your code needs to be used by some Objective-C code, wrap it to expose the desired functionality. Use @objc on individual methods and variables as necessary rather than exposing all API on a class to Objective-C via @objcMembers.

class PriceBreakdownViewController {

  private let acceptButton = UIButton()

  private func setUpAcceptButton() {
    acceptButton.addTarget(
      self,
      action: #selector(didTapAcceptButton),
      forControlEvents: .touchUpInside)
  }

  @objc
  private func didTapAcceptButton() {
    // ...
  }
}

⬆ back to top

Contributors

View Contributors

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Amendments

We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

⬆ back to top

License

captainbarber99/swift

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